Cardiac hypertrophy in humans is frequently associated with a poor prognosis. In cultured rat cardiac myocytes, ligands such as phenylephrine, endothelin-1, angiotensin II, and prostaglandin F2alpha promote a hypertrophic response. There is evidence that the action of these ligands on cultured cells mimics cardiac hypertrophy in humans. Most agonists that cause cultured cardiomyocytes to hypertrophy signal via heterotrimeric G proteins. Regulator of G protein signaling (RGS) proteins are GTPase activating proteins (GAPs) that deactivate asubunits of heterotrimeric G proteins. The main aim of this proposal is to investigate the physiologic role of RGS proteins in the regulation of cardiac hypertrophic growth program. The central hypothesis is that RGS proteins determine the responsiveness of cardiac myocytes to extracellular stimuli, and that RGS gene expression is increased as an adaptive mechanism to limit Gprotein- mediated signal transduction. In the original funding period, we demonstrated that RGS proteins are present in heart, that RGS3 and RGS4 gene expression is responsive to external stimuli, that cardiacspecific overexpression of RGS4 in mice blocks pressure overload- and exercise-induced cardiac hypertrophy, and that RGS4 is a GAP for Gq in the in vivo heart. Despite these findings, many questions remain about the role of RGS proteins in heart disease. In the next phase of this project, we propose to investigate the specific role of RGS2, RGS3, and RGS4, in cardiac myocyte hypertrophy by use of a "knock-down" strategy in cultured cells. Second, we investigate the specific role of RGS4 in pressure overload- and exercise-induced cardiac hypertrophy by use of mice with both whole-body and cardiacspecific targeted disruption of this gene. Third, we investigate the specific role of RGS3 in pressure overload- and exercise-induced cardiac hypertrophy by use of mice with whole-body targeted disruption of this gene. These experiments will help define the role of RGS proteins in the pathogenesis of cardiac hypertrophy and contractile dysfunction and may provide important information for the development of novel therapeutic agents.